The invention relates generally to thermal ink jet printing and, more particularly, to thermal ink jet printheads with closer packing of transistor active circuits formed on the printhead.
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor heater formed over a silicon substrate isolated therefrom by an underglaze layer. The resistor heater is located in the channels near the ink-ejecting nozzles a predetermined distance therefrom. An ink nucleation process is initiated by individually addressing resistors with short (1-10 .mu.second) electrical pulses from transistor drive circuitry preferably located on the same chip to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
One problem with prior art printheads is that the underglaze layer must be thick enough to provide thermal insulation between the silicon substrate and the resistor heater to the extent necessary to divert most of the energy from the electrically addressed heaters into the ink where it forms a vapor bubble. If the energy is directed into the silicon substrate, it can cause temperature variations requiring compensation. The underglaze layer also acts as the field oxide layer in the electronically active components of the chip, providing electrical isolation between transistors in the driver and logic circuitry. U.S. Pat. Nos. 4,947,192 and 5,030,971 disclose ink jet printheads forming active drive matrices on resistor heater substrates which are electrically connected to a plurality of heater resistors. These patents are hereby incorporate by reference. For this purpose, the field oxide layer need be typically less than one-half the thickness required for thermal isolation. A thinner field oxide layer enables closer packing of transistor active areas on the chip.
While it is possible to decouple these two requirements (thermal and electrical isolation) by growing one oxide layer for transistor isolation and a separate oxide layer for heater thermal isolation, the extra mask level and high-temperature processing required adds additional cost to the printhead die.